nilai dari (cos 20derajat +cos 100 derajat + cos 140 derajat)

TriGonoMetRi

cos 20° + cos 100° + cos 140°
= 2 cos ((100+20)/2) cos ((100-20)/2) + cos 140°
= 2 cos 60° cos 40° + cos 140°
= cos 40° + cos 140°
= 2 cos ((140+40)/2) cos ((140-40)/2)
= 2 cos 90° cos 50°
= 0

••
cos 60° = 1/2
cos 90° = 0